Sizing agents are used in the paper industry to impart resistance to aqueous penetrants to paper and paperboard. The primary products used to provide the property of sizing are rosin, alkenyl succinic anhydride (ASA) and alkyl ketene dimer (AKD). As these hydrophobic materials are insoluble in water, they are typically supplied to the paper machine as aqueous dispersions. This facilitates good mixing with the aqueous pulp slurry if they are added before the sheet is formed (referred to as internal sizing); or with the starch solution if they are added to the surface of the sheet at a size press (referred to as surface sizing).
The aqueous dispersions of these hydrophobic materials must have certain characteristics to be useful for sizing. The emulsions must be stable for a time sufficient to get them from the point of manufacture to the paper machine without loss of properties, physical or chemical. Additionally the emulsions must include a means of retaining the hydrophobic particles on the fiber surfaces.
Manufacture can be at the paper mill (on-site emulsification) if the hydrophobic material is hydrolytically unstable (e.g., ASA), or at a location remote from the paper mill. In the case of dispersions produced at a remote location, the products must be sufficiently high in solids to minimize the cost of shipping, and sufficiently stable to be stored for a period of time long enough to allow for shipping and storage at the mill location.
Due to these requirements, the preparation of emulsions of hydrophobic paper sizing agents has been the basis of numerous patents aiming to improve stability and/or sizing efficiency of the product. For example, Edwards et al. teach stable high solids dispersions of ketene dimer by incorporating water soluble carboxylic acids in a standard starch-based stabilization system (U.S. Pat. No. 4,861,376), Blixt et al. disclose dispersions of ketene dimer with improved sizing efficiency by using cationic starches with a higher degree of substitution (U.S. Pat. No. 4,964,915), Aldrich discloses stable dispersions of fortified rosin using cationic aminopolyamide-epichlorohydrin resins for stabilization (U.S. Pat. No. 3,966,654), Lauzon teaches the stabilization of dispersions of fortified rosin (U.S. Pat. No. 5,846,308) and cellulose reactive sizing agents (U.S. Pat. No. 6,315,824 B1) with a coacervate dispersioning agent comprising an anionic component and a cationic component to improve sizing performance. Dumas teaches the post-addition of cationic polymers to dispersions of hydrophobic cellulose reactive sizing agents to enhance sizing efficiency (U.S. Pat. No. 4,317,756), and Varnell (U.S. Pat. No. 6,123,760) discloses the post-addition of hydrophobically modified water-soluble polymers to aqueous dispersions of hydrophobic paper sizing agents to improve stability.
Frolich et al. (U.S. Pat. No. 6,093,217) disclose aqueous dispersions of cellulose reactive sizing agents stabilized with an anionic, hydrophobically modified cellulose derivative to provide improved sizing in paper making furnishes that have a high cationic demand and/or a high content of lipophilic extractives, and/or paper machines with a high degree of closure. In the specification, the hydrophobically modified cellulose derivative is initially referred to as a “hydrophobically modified dispersing agent”, with a long list of possible options provided. It is then stated that the preferred embodiment of this invention also includes a surfactant, which means that the hydrophobically modified cellulose derivative is not functioning as a dispersing agent but instead as a stabilizer. Additionally, no examples are provided with any “hydrophobically modified dispersing agent” other than the anionic hydrophobically modified cellulose derivative.
Conner et al, (U.S. Pat. No. 6,183,550 B1) disclose aqueous dispersions of paper sizing compounds stabilized with a water-soluble dispersant containing “at least two hydrophilic groups and at least one hydrophobic group”, referring to a class of compounds called “gemini surfactants”. These surfactants can be used as received or in combination with starch or other dispersants to prepare aqueous dispersions.
Hydrophobically modified poly(aminoamides) useful as fixative detackifiers for stickies and pitch control in papermaking systems are disclosed by Q-M Gu, et al. (US 2010/014746 A1).